Dissociative adsorption of CO and O 2, on Cr(100), Cr(110), and Cr(111) in the temperature range 300–1175 K

Abstract

Electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED), and Auger spectroscopy have been used to characterize the adsorption of CO and O 2 on Cr(100), Cr(110), and Cr(111) at 300 K and above. No evidence of molecular CO or O 2 was observed on these surfaces at any coverage. For each surface, the vibrational spectra for dissociated CO and oxygen are very similar. Broader EEL spectra are observed on Cr(111) than on Cr(110) or Cr(100), indicative of multiple sites on the very open (111) surface. For O 2 exposures on Cr(110) and at low coverages on Cr(111), similar Cr-O stretching frequencies at ≈ 560–580 cm −1 and at ≈ 420–440 cm −1 are suggestive of similar adsorption sites for oxygen on the two surfaces. These sites are proposed to be the long bridge and three-fold sites on (110) planes of the two surfaces. A four-fold hollow site is consistent with the Cr-O stretching frequency at 520 cm −1 for low coverages of oxygen on Cr(100). Adsorption sites for carbon are the same. Frequency shifts with coverage and temperature are reported for both chemisorption systems. A (6 × 1) LEED pattern is observed after an 825 K anneal for a 0.75 L exposure of CO Cr(110) and a model consistent with the LEED pattern and sites based on EELS data is presented.